This invention relates to a scanning electron microscope used for inspection of a reticle to fabricate a semiconductor integrated circuit, or in particular to a scanning electron microscope for inspecting, without contaminating the reticle surface, the dimensions and shape of the pattern formed on the reticle.
In fabrication of a semiconductor integrated circuit, the photolithography is used to form a circuit having fine shapes of various films on the surface of a silicon wafer. According to this technique, a pattern of a predetermined shape preformed on a quartz substrate is transferred by an exposer to a photosensitive resin (photoresist) film formed on the silicon wafer. The quartz substrate thus formed with the pattern is called a reticle. Generally, the reticle is made of a transparent quartz substrate of molten quartz glass or the like having the surface thereof formed with a pattern of an opaque film of a metal (hereinafter sometimes referred to as “the opaque film pattern”). The reticle is fabricated through the steps of forming an opaque film on a quartz substrate, forming a resist pattern of a photosensitive resin on the opaque film, and etching the opaque film through the resist pattern and forming an opaque pattern.
With the advance of micronization of a semiconductor integrated circuit, the resist pattern and the opaque film pattern formed on the reticle have become more and more difficult to inspect with the conventional inspection apparatus using the light. To overcome this difficulty, a technique using the scanning electron microscope for reticle inspection is under development. This is an attempt of using, for reticle inspection, the conventional scanning electron microscope with the measuring function (hereinafter sometimes referred to as “the measuring SEM”) for dimensional measurement and shape inspection of the semiconductor integrated circuit.
The measuring SEM includes a transport means for transporting a silicon wafer formed with a circuit at a high speed in the vacuum chamber of the scanning electron microscope, a moving means for moving a predetermined point on the silicon wafer with a high accuracy to a point just under the optical system of the scanning electron microscope, a calculation means for processing the specimen image formed from a secondary electron signal based on the secondary electrons released from the silicon wafer and thus calculating the dimensions of a predetermined portion, and a control means for executing the series of operation of each means automatically and continuously according to a predetermined procedure. An application of the measuring SEM to the reticle inspection, however, encounters the problem described below. Specifically, when the primary electron beam to release the secondary electrons is radiated on the reticle portion to be observed, the molecules of hydrocarbon and water existing on the reticle surface or in the surrounding space acquire the energy of the primary electron beam and are polymerized. The hydrocarbon, etc. thus polymerized is deposited on the reticle surface. This is the phenomenon called the contamination and conspicuous especially in the reticle inspection using the conventional measuring SEM. More specifically, the reticle inspection is accompanied by a conspicuous contamination due to the fact that the quartz substrate is charged and polarized by the primary electron beam and the resulting local electric field formed on the quartz substrate induces the molecules of hydrocarbon, etc. The contamination not only is deposited on the opaque film pattern to be observed and changes the dimensions thereof but also reduces the light transmittance of the transparent part of the reticle (the part other than the opaque film pattern), thereby often reducing the fabrication yield of the semiconductor integrated circuit. For using the measuring SEM for reticle inspection, therefore, it is necessary to take a measure to reduce the contamination.
A conventional measuring SEM having an ozone generator in a specimen observation chamber is known (e.g. G. W. B. Schulter et. al., Proceedings of SPIE, Vol. 5567 [2004], pp. 876-886). In this measuring SEM, ozone having a large mean free path is introduced into the specimen observation chamber in a low vacuum. The substances causing the contamination (hydrocarbon, etc.) in the specimen observation chamber are decomposed by reaction with ozone having large energy thereby to purify the interior of the specimen observation chamber.
Also, a conventional technique for purifying the surface of the glass substrate of a liquid crystal display element is known, in which ozone having a small mean free path in the atmosphere or comparatively small energy is caused to act on the glass substrate (Hishinuma Norikore: “2001 FPD Technology Daizen”, published by Electronic Journal, 2000″). According to this technique, a vacuum ultraviolet lamp having a wavelength of not more than 200 nm is turned on in the atmosphere thereby to ozonize the oxygen molecules in the surrounding atmosphere. The vacuum ultraviolet light lamp is turned on in the immediate neighborhood of the glass substrate and therefore the vacuum ultraviolet light is radiated on the surface of the glass substrate. As a result, hydrocarbon, etc. is decomposed not only by ozone but also by the optical energy of the vacuum ultraviolet light and the surface of the specimen is purified.
An attempt to inspect the resist pattern of a photosensitive resin of the reticle under the measuring SEM described above in the reference of Schulter et. al., however, would fail due to the fact that the resist pattern to be observed is decomposed easily by reaction with ozone having a large energy, and therefore the reticle cannot be arranged in the specimen observation chamber during the purification process of the interior of thereof. In this measuring SEM, therefore, the specimen cannot be observed during the purification process, thereby posing the problem of an extremely low reticle inspection rate.
Also, in this measuring SEM, ozone cannot be used in the step of observing the resist pattern formed on the opaque film which is considered essential in reticle fabrication. In other words, this measuring SEM cannot purify the contamination caused in the reticle fabrication process. Further, this measuring SEM poses the problem that ozone is diffused in the specimen observation chamber and therefore the hydrocarbon on the reticle surface playing the controlling role in the contamination cannot be effectively removed. The problem of the conventional measuring SEM, therefore, is that the reticle cannot be inspected with high accuracy due to the contamination of the reticle.
In the technique described in the reference of Hishinuma, on the other hand, the decomposition of the resist pattern can be suppressed due to the comparatively low ozone energy, but the resist pattern is liable to be decomposed by the optical energy of the vacuum ultraviolet light.